A human vertebra has a rearwardly projecting portion known as a spinous process. Bending of the spine can cause the spinous processes of adjacent vertebrae to move with respect to one another (e.g. toward and/or away). This may, in turn, constrict the space in the spinal canal and foramina and, thus, may cause pain. Such constriction, which is known as stenosis, can be treated by inserting a spacer into the space between adjacent spinous processes.
Generally speaking there are two types of spinal stenosis: (1) hard or rigid spinal stenosis or (2) soft or dynamic spinal stenosis. In both cases, spinal stenosis may be caused by excessive growth of tissue due to degeneration, loss of disc height, as well disorders such as spondilolisthesis where the normal relative position and/or orientation of the adjacent vertebrae have been modified.
The most significant difference between the two types of spinal stenosis is that generally speaking dynamic spinal stenosis may be treated with distraction of the vertebra at the affected level while hard stenosis generally requires removal of the tissue that obstructs the spinal canal or foramina at the affected level. In case of tissue removal, the patient generally must accept some loss of stability of the spine. Therefore, it is preferable to increase the stability of the spinal segment by inserting an interspinous spacer in between the adjacent vertebrae to increase the stiffness of the segment and/or to restrict motion of that segment.